The present invention relates to a golf club head, more particularly to an improved structure of a ball striking face.
In the laid-open Japanese patent application No. JP-A-8-224328, an impedance matching theory is disclosed. According thereto, if the PLM frequency of a golf club head matches that of a golf ball, then an energy loss is minimized, namely the energy that the golf ball may receive from the golf club head when struck is maximized and the restitution coefficient may be increased, and therefore, the leap is maximized. Here, the xe2x80x9cPLM frequencyxe2x80x9d of an object is the frequency at which the primary local minimum of the mechanical impedance thereof occurs. Incidentally, the mechanical impedance Z is defined as the quotient F/V of the magnitude of an external force F acting upon a point of a body divided by the resultant velocity V of another point of the same body.
The PLM frequencies of the widely used golf balls are about 1000 Hz. In general, however, the PLM frequencies of conventional golf club heads are higher than 1000 Hz. Thus, it is the best way to decrease the club head""s PLM frequency near to 1000 Hz.
The PLM frequency of a golf club head may be decreased if a face part of the club head is decreased in the thickness and/or the face part is made of a material having a lower Young""s modulus. Here, the face part is a front part of the head, which defines the face for striking a gall ball.
In recent years, on the other hand, wood clubs having a head whose major part is made of a metallic material such as stainless, titanium alloy and the like (hereinafter the xe2x80x9cwood-shaped metal headxe2x80x9d or xe2x80x9cwood-shaped headxe2x80x9d) are widely used. In particular, titanium alloys are at present the mainstream of the wood-shaped metal heads because of their strength and relatively low specific gravity.
There is a tendency for such wood-shaped metal heads to have a weight distribution such that the sole part or heel part is heavier than other parts, and in order to realize such weight distribution without increasing the total weight of the head, the thickness of the face part is minimized. Thus, it is very difficult to further decrease the thickness of the face part.
As to materials having a low Young""s modulus, usually the tensile strength and hardness of such material are low when the Young""s modulus is low. Accordingly, if the face part is made of such material, it is difficult to decrease the thickness of the face part in order to lower the PLM frequency.
It is therefore, an object of the present invention to provide a golf club head in which, by making at least a part of the ball hitting face of a special alloy which has a higher strength and lower Young""s modulus than ever, the PLM frequency of the ball striking face is decreased toward those of the golf balls to improve the restitution coefficient without sacrificing the durability of the face part.
According to the present invention, a golf club head comprises a face part defining a face for striking a golf ball, the face part is at least partially made of an alloy satisfying the following three conditions:
(1) the alloy is composed of at least three different metallic elements whose group numbers in the periodic system (or family numbers in the periodic table of the elements) are at least two consecutive numbers;
(2) a difference in the atomic radius between at least two of the at least three different metallic elements is not less than 10%; and
(3) the heat of mixing of the element that is the major component of the alloy and at least one of the remaining components or the remaining component is not less than xe2x88x9210 kcal/mol.
As to the group numbers or family numbers of the elements, sequential numbers from 1 to 18 are used in the following description. Incidentally, this numbering system corresponds to the other well-known numbering system starting from 1A to 0 as follows.
1(1A), 2(2A), 3(3A), 4(4A), 5(5A), 6(6A), 7(7A), 8(8), 9(8), 10(8), 11(1B), 12(2B), 13(3B), 14(4B), 15(5B), 16(6B), 17(7B), 18(0)